This invention relates to apparatus for exchanging a control rod device mechanism in a nuclear reactor for effecting maintenance or inspection thereof in a nuclear power plant.
Generally, in a boiling water reactor, the control rod drive mechanisms is arranged below the pressure vessel and connected to the bottom thereof through a housing in the boiling water reactor core. Although the control rod drive mechanism is detached from the reactor afrer an operation over a predetermined time interval for periodical inspection in an inspection chamber disposed at an independent place near the reactor, this detaching work requires many workers and involves problems regarding safety work and exposure to radiations for the workers.
FIG. 1 is a schematic elevational view showing conventional apparatus for exchanging the control rod drive mechanisms, in which a cylindrical working chamber 1 for exchanging the control rod drive mechanism is provided below the nuclear reactor and surrounded by a wall 2 generally made of concrete. Supporting members 3 are mounted on the inside peripheral surface of the wall 2 and an annular rail is mounted on the supporting members 3. A platform 5 on which a winch is mounted on the rail 4 to be movable freely and rotatable therealong. The winch is constructed so that a carriage 8 for conveying a control rod drive mechanism 7 is guided and accommodated into the inside hollow portion of the platform 5 from a passage 9 formed through the peripheral wall 2. For this reason, it is required that the bottom surface of the passage 9 should be substantially flat with respect to the bottom surface of the platform 5.
The control rod drive mechanism 7 conveyed into the working chamber 1 is attached by bolts to a housing 10 of the control rod drive mechanism 7 in the reactor pressure vessel, and stud bolts 11 are suspended around this mechanism 7 and the housing 10. Although a plurality of control rod drive mechanisms 7 and stud bolts 11 are actually arranged side by side in the reactor, a minimum number thereof necessary to understand illustrated in FIG. 1.
In the arrangement described above, the control rod drive mechanism 7 is detached in the following manner. At first, only two bolts among a plurality of bolts connecting the control rod drive mechanism 7 to the housing 10 are manually removed. A pin carrying a pulley 12 is fitted into one bolt hole, and into the other bolt hole is fitted a pin attached to the front end of the wire rope 14 wound on a drum 13 of the winch 6. The wire rope 14 passes about pulleys 12 and 16, and a locating member 17 is provided with the pulleys at its lower end and with a projecting member 15 which projects upwardly and is adapted to engage the lower end of the control rod drive mechanism 7 to vertically hold the locating member 17. In this condition, the other bolts fixing the control rod drive mechanism 7 to the housing 10 are all removed and the wire rope 14 on the drum 13 is unwound from the winch 6, thus lowering the locating member 7 and the control rod drive mechanism 7 held thereon. The workers hold the control rod drive mechanism 7 to rotate it by 90.degree. from the position shown in FIG. 1 while it is being lowered and then mount it on the carriage which is then transferred into the inspection chamber through the passage 9.
The attachment of the control fod drive mechanism 7 to the housing 10 is carried out in the reverse order to that described above.
However, the attachment and/or detachment of the control rod drive mechanism 7 are performed directly by workers in the working chamber 1 below the reactor pressure vessel under the condition in which the workers may be exposed to relatively high radiations. Therefore, not only it was dangerous for the workers working in the working chamber, but also many workers are required to exchange the control rod drive mechanisms 7.